FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
A technician troubleshoots a hand-operated auxiliary hydraulic pump used for ground servicing and emergency operation. The pump strokes normally, but no pressure builds in the system, and fluid seems to simply move back and forth within the pump body. Which malfunction would most likely explain this symptom?
- AA stuck-open pump inlet/outlet check valve
- BA failed pressure-reducing valve
- CAn improperly rigged selector valve
- DA leaking accumulator precharge
Show answer & explanationAnswer & explanation
Correct answer: A. A stuck-open pump inlet/outlet check valve
A hand (wobble) pump uses internal check valves to direct fluid in one direction on the intake stroke and the opposite direction on the discharge stroke, building system pressure. If a check valve sticks open, fluid can flow backward within the pump instead of being directed into the system, so strokes produce no net pressure rise.
Why the other options are wrong
- B. A pressure-reducing valve affects a downstream subsystem's pressure, not the pump's basic ability to stroke fluid.
- C. An improperly rigged selector valve would misdirect flow to the wrong actuator, not prevent pressure buildup entirely.
- D. An accumulator precharge leak would affect surge damping, not the pump's ability to build pressure at all.
Hand (Wobble) Pump Check Valves
Internal check valves in a manual auxiliary hydraulic pump that direct fluid flow on intake and discharge strokes, allowing pressure to build with each stroke.
- Two check valves per pump: intake and discharge
- Used for ground servicing and emergency system operation
- A stuck-open check valve causes fluid recirculation with no pressure rise
Memory trick: Wobble pump check valves = 'one-way doors' — stuck open means fluid sloshes with no progress.